Metal-connected particle articles

ABSTRACT

Apparatus and methods for making metal-connected particle articles. A metal containing fluid is selectively applied to a layer of particles. The metal in the fluid is used to form metal connections between particles. The metal connections are formed at temperatures below the sintering temperature of the particles in the layer of particles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of pending U.S. Ser. No.15/764,871, filed Mar. 29, 2018, which itself is a national stage entryunder 35 U.S.C. § 371 of PCT/US2016/015723, filed Jan. 29, 2016, each ofwhich is incorporated by reference herein in its entirety.

BACKGROUND

Producing metal parts continues to be dominated by methods such asmachining and casting. In addition, polymers, especially high strengthpolymers, have replaced metal parts in many applications due to theability to mold polymers in high volume with high detail at low unitcost. In contrast, molding for metals has had some success withapproaches like metal injection molding (MIM). However, for a variety oftechnical and economic reasons, methods of metal molding have notachieved widespread use in forming metal parts. When it comes to formingsmall numbers of complex metal parts, machining remains the go tomethodology. The limitations and costs to producing complex metal partsunderscore the need for improved ways to producing complex metal parts.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various examples of the principlesdescribed herein and are a part of the specification. The illustratedexamples do not limit the scope of the claims. Throughout the drawings,identical reference numbers designate similar, but not necessarilyidentical, elements.

FIG. 1 shows a structure according to one example consistent with thepresent specification.

FIGS. 2A-C show an example structure consistent with the presentspecification at three points. FIG. 2A shows the example structureas-deposited; FIG. 2B shows the example structure after consolidation;and FIG. 2C shows the example structure after sintering.

FIG. 3 shows a structure of particles connected with metal connectionsin one example consistent with the present specification.

FIG. 4 shows a method of forming an article of particles connected bymetal connections in one example consistent with the presentspecification.

FIG. 5 shows a method of forming a consolidated part in one exampleconsistent with the present specification.

FIG. 6 shows a method of forming metal connections between particles inone example consistent with the present specification.

FIG. 7 shows a system for forming a consolidated part in one exampleconsistent with the present specification.

DETAILED DESCRIPTION

For producing small quantities of complex mechanical parts made ofmetal, the default approach has been machining. Machining is askill-intensive method and thus may be costly. Increasingly, polymercomponents can be produced by 3D-printing. 3-D printing however faceschallenges in producing high strength parts suitable for replacing metalparts. Many of the materials that are capable of being 3-D printed lackthe desired mechanical strength. 3-D printing has been used to rapidlyproduce casting molds or “lost wax” materials to speed up the formationof metal parts. Some attempts have been made to adapt the 3-D printingapproach to metals by depositing metal-polymer composites. A part isformed by building up layer after layer of material. Adaptation ofinkjet printing technology has provided precise deposition of multiplematerials as part of a 3-D printing process. After forming, thepolymer-metal hybrid part is then subjected to a high temperatureprocess to burn away the polymer and consolidate the metal part.

A challenge with this approach is that the polymer occupies asignificant fraction of the volume of the printed part in order to holdthe part together. Accordingly, as the polymer is removed, generally byheating the component, the loss of volume can cause shrinkage of thepart. This shrinkage may be non-uniform due to a variety of factors,including temperature uniformity, part dimensions, air escape routes,gravity, etc. The shrinkage makes it difficult to produce finalizedmetal parts with good dimensional control using polymer-metal deposits.One proposed solution is to rough form the parts using a 3-D printedpart, sinter the part, and then finalize the dimensions of the part withmachining operations. This may save on machining time, but still usesthe machining resources. In the end, such a hybrid approach offers somebenefits over machining alone, however it still presents somecomplications.

This specification describes forming a solid part composed of metal orother particles that are bound together at the printing location withmetal connections. The particles retain their original shapes, duringthe initial forming and consolidation, helping to assure dimensionalcontrol of the fabricated part. This is because the temperature used toform the metal connections is below the sintering temperature of theparticles and therefore below the melting temperature of the particles.After the metal connected part is formed, if desired, a high temperaturesintering process can be performed to enhance adhesion between theparticles. Alternately, the high temperature process can be performed toconsolidate some of the empty volume in the part. However, in eithercase the initial metal connections significantly improve the dimensionalstability compared with polymer based approaches as the metalconnections retain their mass during the sintering operation. The metalconnections reduce the dimensional change experienced by the part,allowing the production of detailed features and greater dimensionalcontrol. The initial connections provide greater conformance between theconsolidated shape and a post-sintering shape. The use of metalconnections also reduces the amount of mass lost during sinteringoperations. This reduces the shrinkage of the part. Finally, without theoutgassing of the volatilized polymer and decomposed by-products, theissues of gas flow and gas escape routes are avoided.

Accordingly, the use of metal connections between the particles, insteadof deposition of polymer-metal composite, provides advantages indimensional stability of the part as the metal connections do not needto remove significant material during a secondary sintering operation.In contrast, the polymer binder of the polymer-metal hybrid parts isdecomposed and removed. This results in significant dimensionalinstability as the green parts (formed parts) are sintered. The greaterpercentage of the material that is removed during the sinteringoperation, the greater change in dimensions of the part. In manyexamples, this reduction in dimension due to binder mass loss isnon-uniform or asymmetrical causing warping, distortion, or otherwisehindering the ability to form a part with a predictable geometry aftersintering. In contrast, the approach described in this specificationavoids much of this because the connections between the particles areformed with metal. The metal may have a similar melting point to theparticles. The metal connections help stabilize the particle structureduring the secondary sintering. The metal connections also do notexperience the large loss of mass.

As used in this specification and the associated claims, a consolidatedpart is a part composed of particles bound together by metal connectionswhere the particles retain their original shape.

As used in this specification and the associated claims, a final part isa consolidated part that has been subjected to a secondary sinteringoperation in order to increase the adhesion between particles. Theparticles in a final part may have their shape modified from theiroriginal shape due to the sintering operation.

Accordingly, the present specification describes, among other examples,a method of forming an article, the method comprising: forming a layerof particles; selectively applying a fluid containing a metal to thelayer of particles; and forming connections between the particles usingthe metal in the fluid.

The present specification also describes a method of forming an article,the method comprising: selectively depositing metal nanoparticles onto alayer of particles and fusing the metal nanoparticles to form aconsolidated part.

The present specification also describes an apparatus for forming aconsolidated part. The apparatus comprises: a spreader to form a layerof particles; a printing device to selectively deposit ametal-containing fluid onto a portion of the layer of particles; and aheater to form metal connections between the particles of the layer ofparticles using the metal in the metal-containing fluid.

Turning now to the figures:

FIG. 1 shows a structure according to one example consistent with thepresent specification. In the structure, a number of particles (120) areconnected together with metal connections (130). The metal connections(130) are formed at a temperature below the sintering temperature of theparticles (120).

The structure may be assembled on a substrate. The substrate may be atemporary substrate or a permanent substrate. The substrate may includea heating element to facilitate the initial consolidation of the metalconnections (130). The substrate may be substantially flat. Alternately,the substrate may include features to facilitate shaping the part beingformed. Examples of such features include, but are not limited to:groves, holes, bumps, protrusions, prominences, guide features,alignment features, etc. The substrate may be coated with a thin layerof mold release in order to prevent adhesion between the part and thesubstrate.

Heating may alternately and/or additionally be provided by an overheadsource. The heat may be applied across the entire part being formed ormay be localized to the area being currently built up. The heat may beprovided using any appropriate equipment, for example, IR, NIR, UV orvis lamp, flash-lamp, etc. Heat may be applied from just the substrate.Heat may be applied using just an overhead source. Heat may be appliedusing a combination of methods and locations. In one example, the systemalso includes a temperature sensor. The temperature sensor may be acontact sensor or a non-contact sensor, e.g., an IR sensor. Thetemperature sensor may report the temperature of the part underassembly. The temperature sensor may report the temperature of otherportions of the printing system. In one example, a non-contact IR sensoris mounted to the printhead and directed toward the deposition location.A non-contract IR sensor may be located near a heat source and directedtowards the target of the heat source.

The particles (120) as used in this specification encompass a wide rangeof shapes, sizes, distributions, materials, etc. Whatever shape, size,distribution, or material is used, the particles retain their shapeduring the process of applying them to the developing part and formingthe metal connections (130) between the particles (120). Severaltechniques for forming metal connections (130) below the sinteringtemperature of the particles (120) are described below.

The particles (120) may be of any shape. The particles (120) may beformed by processing. The particles (120) may be naturally shapedmaterials. The particles (120) may include a variety of differentshapes. The particles (120) may be selected and/or sorted to have agiven geometry. The particles (120) may include flakes, sheets, plates,or similar flattened, primarily two-dimensional geometry. The particles(120) may include rods, spindles, spheres, blocks, etc. The particlesmay have a mean and/or median size of 1 to 1000 micron in their largestdimension.

The particles (120) used may be of a single size distribution. Theparticles (120) may include a mixture of multiple size distributions.The particles (120) may all have a similar and/or identical composition.Alternately, several different types of particles (120) may be combined.For example, multiple types of metal particles (120) could be combinedto form an alloy after a secondary heating operation. Alternately,composition gradients can be formed. For example, areas of a partneeding ductility may have a higher nickel concentration. By comparison,nickel concentration may be at a reduced level near surfaces that willbe in contact with skin. The formation of structured/supportedelectrodes can be formed. For example, a small area of a noble metalsuch as platinum or gold can be formed in an area of a refractory metalsuch as titanium. The refractory metal can be oxidized in a post-formingprocess to form an insulator leaving the noble metal area to act as anelectrode. Because both the particles (120) and the deposited metal canbe selected and applied with high precision, a wide variety ofnon-uniform materials can be readily formed. Thus, composition gradientsin the consolidated part can be formed using variation of the particles,variation of the deposited metal connections, or both.

The particles (120) as used in this specification may be formed of awide variety of materials including, but not limited to: metal, metaloxides, metal carbides, metal nitrides, ceramics, non-metals,metalloids, semiconductors, polymers (especially thermosets butincluding thermoplastic polymers), minerals, carbon black, graphite,diamond, and organic materials. The particles (120) are stable duringthe formation of the metal connections. As this may be accomplished in avariety of ways described below, different materials will be more and/orless suited for particular approaches. Further, the material selectionhas a significant impact on the characteristics of the consolidated partand what kinds of secondary processes may be used. Clearly, theparticles selected will also impact the characteristics of a final part.

The particles (120) are spread as a layer on the substrate (110). Thelayer may be uniform. Alternately, the layer may have areas ofdifferential thickness to adjust the shape of the developing part.During or after consolidation of the part in the building location, theexcess, unattached particles can be removed and recycled. The excessparticles may be removed mechanically, for example, using brushes.Alternately, an air jet, fluid jet, or ultrasonic cleaner may facilitateremoval. The substrate (110) can be a previously formed particle (120)layer. In this manner, the total thickness of the part can be built upto form the desired thickness of the consolidated part.

The metal is selectively applied to the layer of particles (120). Metalis applied in areas where the particles will be used to form the finalarticle and is not applied in areas that will not be part of the finalarticle. However, as in any manufacturing operation, there may beadvantages in forming temporary structures to stabilize or support thedeveloping article that are removed in subsequent operations. While themetal connections (130) between particles (120) can be formed by anysuitable method, specific examples will be discussed to show how thismay be accomplished. Further, the metal is not formed by melting and/orsintering the particles (120). The metal used to form the metalconnections (130) is applied as part of an ink. The ink may then beactivated to form the metal connections (130) between the particles(120).

FIGS. 2A-C show a structure consistent with the present specification atthree points in the processing of the part. FIG. 2A shows a structureas-deposited; FIG. 2B shows the structure as a consolidated part; andFIG. 2C shows the structure after a secondary sintering.

FIG. 2A shows the substrate (110) and particles (120). Also, shown aremetal nanoparticles (240). The elements depicted in FIG. 2A, are not toscale. Metal nanoparticles (240) have the property of a significantlyreduced melting temperature compared to larger particles or bulkmaterials (Journal of Materials Chemistry C, 2013, 1, 4052).Accordingly, the metal nanoparticles (240) also have a similarly reducedsintering temperature. This provides the ability to form a material thatcan be thought of as a composite of a single material, as thenanoparticles (240) and the larger particles each have distinct meltingand sintering properties. The nanoparticles (240) function as the sourceof the metal connections between the particles. The nanoparticles (240)can be sintered or modified by temperatures that have no impact on thelarger particles (120), even larger particles (120) of the samematerial. Thus, the substrate (110) can heat the particles (120) to, forexample, 400° C. melting and/or reorganizing the nanoparticles (240).

Because the nanoparticles (240) are deposited in an ink, they will tendto concentrate in the areas between the particles (120). This is becauseas the fluid in the ink evaporates, the droplets will minimize theirsurface area. This will tend to place them at areas of contact orproximity between adjacent particles (120) as such areas tend to allowvery high volumes to new surface area, with the advantage of wetting thesurfaces and obtaining a reducing in free energy. The result is that thedeposited solids in the inks will tend to concentrate at points ofcontact between particles (120). Heating the deposited solids causes thenanoparticles (240) to reorganize under a sintering regime, or as thetemperature is increased the nanoparticles (240) will melt. As thenanoparticles (240) reorganize and lose their high surface area tovolume, they may solidify. Alternate, the part may cool to solidify thematerial.

In another example, the metal nanoparticles (240) have their stabilitydisrupted by adding a second chemical, for example, sodium chloride.This renders the nanoparticles (240) unstable and causes them to depositfrom the solution, forming the metal connections between the particles(120).

In another example, the deposited ink includes a metal salt. Heating ofthe deposited ink causes the solvent to evaporate and the metal salt todecompose forming metal connections between the particles (120).

Nanoparticles (240) can be formed from a wide variety of metals.However, the surface of some metals tends to oxidize, forming metaloxides. In some examples, it is advantageous to coat the metalnanoparticles (240) to prevent or reduce the surface oxidation. Thecoating may be of a second metal. For example, an iron nanoparticle(240) could be coated with silver to avoid oxidation. The coating may bea polymer. For example, a nickel nanoparticle could be coated withpolyethylene. The coating may be a suitable organic or inorganic coatingthat reduces and/or prevents the oxidation of the coated metalnanoparticle (240). In some examples, the coating is designed todecompose or volatilize at temperatures under the melting point of thenanoparticle. This may help prevent the coating from inhibiting adhesionbetween the nanoparticle (240) and the adjacent particles (120). Inanother example, metal oxide coated nanoparticles are provided with areducing agent either as part of the particle, as a coating, or appliedseparately. The reducing agent is activated causing a reduction of themetal oxide and formation of metal connections. The coating may be usedto apply an electrostatic charge to the nanoparticles (240). An oppositecharge may be applied to the substrate (110) to enhance penetration bythe coated nanoparticles (240) into the particle (120) layer. This mayallow thicker layers of particles (120) and reduce the number of passesto build a given thickness part.

FIG. 2B shows the consolidated part with the nanoparticles (240)reorganized to form metal connections (130) between the particles (120).The metal connections (130) preferentially occupy spaces between theparticles (120) due to the high volume to new surface area ratioprovided by those locations. However, not every junction need beconnected in order to form a solid, consolidated part. There is a randomcomponent to which particles (120) get connected together and which donot. Accordingly, there is a threshold below which the weight percentageof nanoparticles (240) will not provide acceptable strength. In someinstances, a part with a lower nanoparticle (240) loading may be brittleor vulnerable to damage even though it is solid. However, asnanoparticles (240) tend to be expensive compared with the particles(120), there is a tradeoff between mechanical robustness and thepercentage of nanoparticles (240) used as a percentage of total weightof the part.

Functional parts have been produced with compositions of 0.5, 1, 3, andup to 15 wt. % nanoparticles (240) by weight of the final part. Thesetests have shown adequate strength to enable handling and clearing ofthe consolidated part in a silver nanoparticle (240)—copper particle(120) system. Higher loadings of nanoparticles (240) appear to reducethe temperature needed to obtain consolidation, produce higherdensities, and may reduce the shrinkage observed during a subsequentheating/sintering operation. This data is consistent with a model wherethe nanoparticles (240) are binding the particles (120) together andfilling gaps between adjacent particles (120). In such a model, highernanoparticle (240) loads need a lower percentage of the nanoparticles(240) to form connections. This accommodates a lower temperature, asnanoparticles (240) have a size distribution and smaller particles willmelt at lower temperatures. Similarly, the greater volume of gapsbetween particles (120) that is filed by the nanoparticles (240), theless volume of air remains in the part to drive contracture duringsubsequent thermal processing.

The application of pressure to the particle (120) layer increases thedensity of the consolidated part. This also increases the density of thefinal part. The use of ultrasonication or vibration may increase thedensity of the consolidated part. The use of a bimodal distribution ofparticles with both large and small particles may increase the densityof the consolidated part.

A variety of formulations can be applied to the particles to form themetal connections. Because these formulations are easily deposited usingan inkjet, they are referred to as inks. However, as used in thisspecification and the associated claims, the term ink refers to asolution that is capable of being ejected from an ink jet, for example,in a thermal inkjet (TIJ) printing device and/or a piezoelectric inkjet(PIJ) printing device. That is to say, in this specification, the termink does not indicate the ability to color the material onto which theink is deposited.

In many of the following ink formulations, one referenced component isan ink vehicle. Ink vehicle is not used in all the ink formulations. Inkformulations without ink vehicle also work as demonstrated by the rangeof ink formulations below. An ink vehicle is a premix that facilitatescompatibility with inkjet printing, especially with thermal inkjetprinting. An ink vehicle may include water, co-solvent(s), pHadjuster(s), and/or surfactant(s). The aqueous formulation may alsoinclude other additives, such as a biocide and/or an anti-kogationagent.

Examples of suitable co-solvents include 2-pyrrolidinone,N-methylpyrrolidone, 1-(2-hydroxyethyl)-2-pyrrolidinone, 1,6-hexanediolor other diols (e.g., 1,5-Pentanediol, 2-methyl-1,3-propanediol, etc.),triethylene glycol, tetraethylene glycol, tripropylene glycol methylether, or the like, or combinations thereof. Whether used alone or incombination, the total amount of the co-solvent(s) ranges from about 1wt. % to about 60 wt. % of the total weight of the ink vehicle.

Examples of suitable surfactants include a self-emulsifiable, nonionicwetting agent based on acetylenic diol chemistry (e.g., SURFYNOL® SEFfrom Air Products and Chemicals, Inc.), a nonionic fluorosurfactant(e.g., CAPSTONE® fluorosurfactants from DuPont, also referenced as ZONYLFSO), and combinations thereof. In other examples, the surfactant is anethoxylated low-foam wetting agent (e.g., SURFYNOL® 440 or SURFYNOL®CT-111 from Air Products and Chemical Inc.) or an ethoxylated wettingagent and molecular defoamer (e.g., SURFYNOL® 420 from Air Products andChemical Inc.). Still other suitable surfactants include non-ionicwetting agents and molecular defoamers (e.g., SURFYNOL® 104E from AirProducts and Chemical Inc.) or water-soluble, non-ionic surfactants(e.g., TERGITOL™ TMN-6 from The Dow Chemical Company). In some examples,it may be desirable to utilize a surfactant having ahydrophilic-lipophilic balance (HLB) less than 10.

Whether a single surfactant is used or a combination of surfactants isused, the total amount of surfactant(s) in the ink vehicle may rangefrom about 0.5 wt. % to about 1.5 wt. % of the total weight of the inkvehicle. pH adjusters may be used to control the pH of the ink vehicle.For example, the pH adjuster may constitute up to about 2 wt. % of thetotal weight of the ink vehicle.

Examples of suitable biocides include an aqueous solution of1,2-benzisothiazolin-3-one (e.g., PROXEL® GXL from Arch Chemicals,Inc.), quaternary ammonium compounds (e.g., Bardac® 2250 and 2280,Barquat® 50-65B, and Carboquat® 250-T, all from Lonza Ltd. Corp.), andan aqueous solution of methylisothiazolone (e.g., Kordek® MIX from TheDow Chemical Co.). The biocide or antimicrobial may be added in anyamount ranging from about 0.1 wt. % to about 5 wt. % with respect to thetotal weight of the ink vehicle.

An anti-kogation agent may be included in the ink vehicle. Kogation isthe deposit of dried material on a heating element of a thermal inkjetprinthead. Anti-kogation agent(s) assist in preventing the buildup ofkogation. Examples of suitable anti-kogation agents includeoleth-3-phosphate (e.g., commercially available as CRODAFOS™ O3A orCRODAFOS™ N-3 acid from Croda), or a combination of oleth-3-phosphateand a low molecular weight (e.g., <5,000) polyacrylic acid polymer(e.g., commercially available as CARBOSPERSE™ K-7028 Polyacrylate fromLubrizol). Whether a single anti-kogation agent is used or a combinationof anti-kogation agents is used, the total amount of anti-kogationagent(s) in the ink vehicle may range from about 0.1 wt. % to about 5wt. % based on the total weight of the ink vehicle.

An example of an ink vehicle is: 1 wt. %-50 wt. % 2-pyrrolidinone, 0.1wt. %-5 wt. % anti-kogation agent, 0.1 wt. %-5 wt. % biocide, 0.01 wt.%-5 wt. % other additives, balance DI water.

All percentages are expressed as weight percent of the totalformulation, except when specifically noted otherwise, for example, aspart of a component of the final solution.

Ink formulation 1: 67 wt. % silver nanoparticle ink (ink is 40 wt. %silver), and 33 wt. % ink vehicle. An example of the silver nanoparticleink is JS-B40G from Novacentrix. This ink can be applied with a TIJ andhas a fusing temperature of approximately 200° C. with 250° C. resultingin full mechanical properties.

Ink formulation 2: 100 wt. % silver nanoparticle ink. An example isMitsubishi Silver Nanoparticle Ink NBSIJ-FD02. This ink can be appliedwith a TIJ and has a fusing temperature of approximately 140° C.

Ink formulation 3: 14 wt. % copper (II) sulfate pentahydrate, 53 wt. %water, 33 wt. % ink vehicle. This ink can be applied with a TIJ and hasa fusing temperature of approximately 120° C.

Ink formulation 4: 20 wt. % copper (II) nitrate trihydrate, 80 wt. %water. This in has a fusing temperature of approximately 120° C.

Ink formulation 5: 20 wt. % iron (II) sulfate heptahydrate, 80 wt. %water. This ink has a fusing temperature of approximately 120° C.

Ink formulation 6: 20 wt. % iron (II) nitrate pentahydrate, 80 wt. %water. This ink has a fusing temperature of approximately 120° C.

Ink formulation 7: 10 wt. % copper (II) acetate hydrate, 90 wt. % water.This ink has a fusing temperature of approximately 150° C.

Ink formulation 8: 10 wt. % iron (II) acetate, 90 wt. % water. This inkhas a fusing temperature of approximately 150° C.

Ink formulation 9: 100 wt. % copper nanoparticle dispersion. Forexample, copper nanoparticle dispersion from Promethean Particles. Thisink has a fusing temperature of approximately 100° C.

Ink formulation 10: 3 wt. % sodium chloride, 77 wt. % water, 33 wt. %ink vehicle. This ink can be applied with a TIJ. The salt serves todestabilize nanoparticles causing them to leave solution and form solidmetal. This reduces the temperature to form the metal connections.

Ink formulation 11: 100 wt. % copper oxide (CuO) nanoparticle ink. Forexample, ICI-002 from Novacentrix.

Ink formulation 12: 100 wt. % iron oxide nanoparticle ink (20 wt. %Fe₃O₄ in water). For example, product 16809 from US ResearchNanomaterials, Inc.

FIG. 2C shows the part after heating to a temperature sufficient toallow diffusion and/or reorganization of the adjacent particles (120).This is a secondary sintering operation. It is not a requirement inorder to form a consolidated part. Indeed, the metal connections (130)formed between the particles render the consolidated part solid. Thissecondary sintering operation may be performed after the unconnectedparticles are removed from the consolidated part. The consolidated partmay be removed from the substrate (110) prior to the sinteringoperation. This may facilitate removal of unconnected particles (120).In some examples, the substrate (110) is maintained during the sinteringoperation and the final part is removed from the substrate (110) aftersintering.

During sintering, the particles (120) may adjust shape. Diffusionbetween adjacent particles (120) allows the formation of strongmechanical connections. The sintering process can be used to improveadhesion between the particles (120). Sintering can also be used toincrease the density of the part. For example, small voids in thestructure may be filled with material from adjacent particles (120).Diffusion sintering regimes and remodeling sintering regimes are withinthe knowledge of the skilled practitioner and useful texts are availableto provide guidance and models for such operations, for example: RandallGerman, 1994, Metal Powder Industries Federation, Princeton, N.J.Finally, because the actual sintering temperature and mechanismtemperature thresholds depend on the materials selected for theparticles (120) and nanoparticles (240), some experimentation, guided bythe melting points of the particles (120) and the examples below, may beneeded to optimize the specific part compositions and geometriesselected. Three examples of post-consolidation sintering methods are nowdescribed:

Inert gas method: cleaned consolidated parts, consisting of ˜10 wt. %silver from ink 1 and ˜90 wt. % copper from copper particles, are placedinto a muffle furnace which is closed and evacuated. The chamber is thenfilled with nitrogen gas. The furnace is heated from room temperature to975° C. over 300 minutes and then kept at 975° C. for 90 minutes. Thefurnace was allowed to cool to room temperature, after which the partswere removed.

Forming gas method: cleaned consolidated parts, consisting of ˜10 wt. %silver from ink 1 and ˜90 wt. % copper from copper particles, are placedinto a muffle furnace which is closed and evacuated. The chamber is thenfilled with forming gas (4% hydrogen, 96% nitrogen). The furnace is thenheated from room temperature to 900° C. over 600 minutes and then keptat 900° C. for 60 minutes. The furnace was allowed to cool to roomtemperature, after which the parts were removed.

Carbon packing method: cleaned consolidated parts, consisting of 10 wt.% silver from ink 1 and ˜90 wt. % copper from copper particles areembedded within activated carbon pieces held within an alumina crucible.At least 1 cm of activated carbon separates the consolidated part fromthe crucible on all sides. The crucible is covered with an alumina coverand places in an ambient environment furnace. The furnace is brought to930° C. over 120 minutes and then kept at 930° C. for 240 minutes. Thefurnace was allowed to cool to room temperature, after which the partswere removed.

The phrase cleaned in reference to the consolidated parts indicates thatundesired, non-connected particles (120) may be removed from theconsolidated part. In some examples, temporary support structures usedduring forming are also removed prior to the secondary sinteringoperation. In other examples, the support structures are removed afterthe secondary sintering operation.

FIG. 3 show an example of a fused part formed using another metaldeposition method according to the present specification. The substrate(110) is shown with a collection of particles (120). The particles (120)are connected by deposited metal (360).

One method of forming a part consistent with FIG. 3 is to deposit afluid containing a reducing agent and a metal oxide in the liquidcarrier using a printhead. The printhead may be a thermal inkjet (TIJ)or a piezoelectric inkjet (PIJ). In one example, the firing pulse of theTIJ is used to activate a chemical process in the ejected fluid.

Another method of forming a part consistent with FIG. 3 is toselectively deposit a fluid containing a reducing agent onto a layer ofmetal oxide particles. The reducing agent is the activated to connectthe adjacent particles together to form the consolidated part. Theconsolidated part is bonded together with metal connections. Theintermediate form may then be subjected to a secondary sintering processto produce a final part. Alternately, the reducing agent may be appliedalong with a metal oxide particle containing ink, such as inksformulations 11 and 12, listed above. This can be accomplished byloading the metal oxide particle ink in one ejector and the reducingagent in a second ejector on a printhead.

Another method of forming a part is to deposit a solution of metalsalts. The solution is evaporated and the metal salt decomposed to formthe metal connections (130) between the particles (120). Examples ofsuch salt solutions include inks 3-8 which have been used to performthis process on stainless steel and copper particle (120) layers.

Another method of forming a part is to deposit a solution containingmetals in an organic complex or chelated with a decomposable chelatingagent. The organic component is preferentially one that can bedecomposed and at a lower temperature in order to facilitate formationof metal connections between the particles (120). The metal may also besuspended in an emulsion or sequestered in a separate phase usingmicelles which are then disrupted by chemical, thermal, or otherprocesses to cause the precipitation of the metal. As the fluid carrierswill tend to occupy the areas of maximum volume to minimum new surfacearea, this will tend to maximize the deposition in areas where theparticles (120) are in contact or close proximity.

The following description provides a description of a method of formingparts using a thermal inkjet printer consistent with this specification.A layer of particles is rolled out in the target area of the printer.The particles may be commercially sourced. For example, stainless steelpowder is available as product 088390 from Alfa Aesar. Similarly, copperpower is available as product 326453 from Sigma Aldrich. The ideal layerthickness depends on the particle size, the ink being used, thetemperature of the particle layer, etc. However, a layer thickness ofapproximately 100 micrometers has proven workable.

The printer has an appropriate ink loaded in the printhead. The ink isapplied to the particle layer. Multiple inks can be used. Each ink mayinclude different metals and/or facilitate different mechanisms of metalconnection (130) formation. After the ink has been applied to thedesired portions of the layer, a heat lamp can heat the particle layerto the fusing temperature, producing the metal connections between theparticles (120). In some examples, the particle (120) layer ismaintained at the fusing temperature during application of the ink. Theparticle (120) layer may be at a higher temperature than the fusingtemperature of the ink. There is a tradeoff between fusing time andpenetration depth by the ink. If the particle (120) layer is too hot,the ink will tend to fuse on the surface and the thickness of theparticle layer (120) should be kept thin in order to facilitate goodstrength within the consolidated part. Similarly, if the temperature ofthe particle layer is too low, then the fusing time may be excessivelylong or the fusing may be inadequate for good strength. Accordingly,some optimization may improve results once the specific ink and particlemixture have been determined. The inks described above have listedfusing temperatures.

A new layer of particles is applied on top of the previous layer. If theprevious fusing temperature was high, the new layer may be allowed tocool, for example to approximately 150° C. prior to the application ofink to the new layer. The process is repeated until a suitable thicknesshas been built up. The part is allowed to cool and the unconnectedparticles are removed and recycled. The use of a single type ofparticles (120) makes recycling easy. When forming gradient parts withmultiple kinds of particles (120), additional methods such as magneticseparation may help in recovering the unconnected particles (120). Ifthe part is to be used as-consolidated then it is done. Alternately, thepart can be subjected to a sintering operation to further consolidateand/or densify the part.

FIG. 4 shows an example of a method (400) consistent with the presentspecification. The method (400) comprises: forming a layer of particles(410), selectively applying a fluid to the layer of particles, whereinthe fluid comprises a metal (420), and consolidating portions of thelayer of particles, wherein consolidation causes the metal in the fluidto connect together particles in the layer of particles (430). Each ofthese operations is discussed in further detail below.

Forming a layer of particles (410) allows rapid placement of the massused to build the part. Unlike deposition methods where all the mass isprovided through a printer jet, most of the mass is provided in thelayer of particles. This dramatically reduces the amount of materialprovided by the printhead which impacts costs and material handling. Thelayer can be formed in a variety of methods including spray, spreading,air jets, piping, and mechanical methods.

In one example, the layers are pre-formed sheets held together by ameltable and/or sublimatable material, such as ice or dry ice, and thelayers are simply applied and dried before application of the ink. Thepre-formed layers may include an active species that interacts with thedeposited ink.

The part being formed may be mounted on a moving base. After a givenlayer is completed, the base is lowered and the top of the part becomesrecessed compared to a particle collection area. A flat edge is thenpassed over the developing part, applying a layer of particles, with theexcess particles (120) moved to the particle collection area forapplication in a subsequent pass or part. When the part is complete, thebase is elevated and the consolidated part removed. Such an arrangementkeeps the distance between the printhead and the current layer constant,provides support on the sides for the previous layers, including theareas of the previous layers that were not consolidated, facilitatesrapid placement of the particle layer, and aids handling and recyclingof the particles (120) efficient with minimum automation costs.

Selectively applying a fluid to the layer of particles (120), whereinthe fluid comprises a metal (420) can be accomplished in a number ofways. One method is to use a printhead to apply the solution to portionsof the layer of particles while not applying the solution to otherportions of the layer of particles. In another example, a stencil orshield may prevent application to some areas while allowing applicationto other portions of the layer. The process may be automated,semi-automated, or manual. The portions of the layer may be formed intovoxels that are treated individually or several voxels may be treatedsimultaneously. A single metal containing solution may be used for alltreated portions of the layer or multiple metal containing solutions maybe used for different parts of the layer, for example to form differentcompositions, gradient compositions, a tailored microstructure, and/orprovide different mechanical properties in different locations of thefinal part.

As an alternative to depositing material using a thermal inkjet (TIJ)printing device or a piezoelectric inkjet (PIJ) printing device, anaerosol jet, a dropper, or similar component can be used to depositeither droplets or solid material as part of the forming process.

Consolidating portions of the layer of particles, wherein consolidationcauses the metal in the fluid to connect together particles in the layerof particles (430) can similarly be accomplished by a variety ofmethods.

One approach is the use of metal nanoparticles in the solution.Nanoparticles (240) can have melting points and sintering temperatureshundreds of degrees below the temperature of the bulk material and mayinclude micron sized particles. This allows nanoparticles (240) to meltor sinter and connect larger particles (120) without heating theparticles (120) to high enough temperature to cause bulk melting orsintering. Applying a smaller temperature change limits the expansionand contraction effects on the part. Applying a smaller temperaturechange makes the cycle time short. The large melting point separationbetween the nanoparticles (240) and the particles (240) providessignificant system robustness to non-uniformity in heating. The abilityto use two or more sizes of nanoparticles (240) with districttemperature transitions allows even more control over the connectionforming operation. Nanoparticles (240) can be of the same material asthe particles, which allows the formation of an article with a singlecomposition. Alternately, the nanoparticles (240) can be used to createdifferent compositions in different areas of the formed part. It thenanoparticles (240) are providing this differentiation, then this can beaccomplished using a printhead with multiple solutions, where thedifferent ejectors of the printhead provide different solutions.

Another approach is to precipitate the metal from a solution as part ofevaporating the carrier fluid. As the amount of carrier fluid decreases,the concentration of the components in the solution increases. At somepoint the solution becomes saturated and potentially supersaturated. Atthis point, further evaporation produces precipitation of the metal inthe form of metal salts or metal compounds. The metal salts can then bethermally or chemical decomposed to form metal connections. Theevaporating droplets will tend to concentrate at contact points ofadjacent particles (120) and between adjacent surfaces where the amountof surface area (and the free energy associated with it) is minimized.This will result in the deposited material being deposited at thecontact points of the particles (120) where it will tend to hold themtogether. The deposited material may decompose under temperature orchemical processes to increase the strength of the connections betweenthe particles (120). In one example, the deposited material is a metaland a chelating agent. In another example, the deposited material is ametal-organic complex. Alternatively, a secondary solution that may notcontain a metal can be deposited to chemically modify the metal depositfor forming metal connections.

FIG. 5 shows an example of a method (500) consistent with the presentspecification. The method (500) comprises: selectively depositing (510)metal nanoparticles (240) onto a layer of particles (120) and fusing(520) the metal nanoparticles (240) to form a consolidated part.

The operation of selectively depositing (510) the metal nanoparticles(240) onto a layer of particles (120) can be accomplished by loading ametal nanoparticle containing ink into a printhead or ink cartridge thatis attached to a printhead. The printhead is then used to apply thenanoparticles (240) to a layer of particles (120) located in theprinting area. In this approach, the carrier fluid that makes up part ofthe printing ink may be evaporated away, leaving the nanoparticles (240)in the desired locations. Non-limiting examples of suitable inks includeink formulations 1, 2, 9, 11, and 12.

The operation of fusing (520) the metal nanoparticles (240) to form aconsolidated part can be accomplished with a number of previouslydescribed techniques. For example, the particle (120) layer can beheated to produce sintering and/or melting of the nanoparticles.Alternately, an agent such as ink 10 can be applied to destabilize theligands that allow the nanoparticles (240) to remain stable in solution.The nanoparticles (240) then leave solution and form metal connectionsbetween the particles (120).

FIG. 6 shows an example of a method (600) consistent with the presentspecification. The method (600) comprises: forming a plurality of metalconnections between a plurality of particles wherein the plurality ofmetal connections are formed at below a sintering temperature of theplurality of particles (610).

Operation (610) can be performed used the techniques discussed above.Forming the metal connections at below the sintering temperature of theparticles (120) helps provide stability to the structure as theparticles are not remodeling during the forming operation. Further, if asecondary sintering operation is performed, the metal connections serveto stabilize the part.

FIG. 7 shows an apparatus (700) for forming a consolidated partaccording to one example of the present specification. The apparatus(700) comprises: a spreader (795) to form a layer of particles (120); aprinting device (770) to selectively deposit a metal-containing fluid(780) onto a portion of the layer of particles (120); and a heater (790)to form metal connections between the particles (120) of the layer ofparticles (120) using the metal in the metal-containing fluid (780).

The apparatus (700) may be used on a printer such as a 3-dimensionalprinter. The apparatus (700) may include additional components.

The spreader (795) forms a layer of particles (120). The spreader (795)may deposit the particles (120). The spreader (795) may push theparticles (120) to form the layer of particles (120). In one example,the spreader (795) includes a flat blade that is moved across the areawhere the layer of particles is formed. In other examples, the spreadermay have a V-shaped or wedge-shaped blade. In yet another example, thespreader (795) may have a sloped blade. The spreader (795) may have afeed associated with the blade to deposit particles in front of theblade. The spreader (795) may include multiple feeds. The spreader (795)may move in response to a signal associated with a control on theapparatus (700). For example, the apparatus (700) may include a buttonthat, when pressed, causes the spreader (795) to operate. The operationof the spreader (795) may be controlled by a processor, for example, aprocessor that also controls operation of the printing device (770).

The spreader (795) may function without a blade and without mechanicalcontact with the particles (120). For example, the spreader (795) mayshake or otherwise distribute (120) particles over an area to form thelayer. The spreader (795) may include air or other jets to disperseparticles (120). The spreader (795) may include a vibrating device tolevel and densify the particles (120) into a layer. The spreader (795)may include selective feeds to distribute multiple kinds of particles(120) while forming the layer. In one example, the spreader (795) iscapable for forming fine distributions of multiple types of particles(120) through selective placement.

The printing device (770) may be an ink jet printing device. In oneexample, the printing device (770) is a thermal inkjet (TIJ) printingdevice. In another example, the printing device (770) is a piezoelectricinkjet (PIJ) printing device. The printing device (770) may be an air orfluid pressure based ejection system. The printing device (770) may bepart of a printhead. Such a printhead may include a large number ofprinting devices (770) to eject a single fluid (780). The printingdevices (770) on the printhead may eject multiple types of fluid (780).

The fluid (780) includes metal. The fluid (780) may be one of the inksformulations described in this specification. For example, the metal inthe fluid may be in the form of metal nanoparticles (260). The fluid(780) may include components designed to facilitate operation of theprinting device (770). The fluid (780) may contain water. The fluid(780) may be a water-free composition. In one example, the fluid (780)is primarily water, with the metal nanoparticles comprisingapproximately 2 wt. % to approximately 60 wt. % of the fluid by weightof the total fluid. The fluid (780) may include a chemical species thatis activated by the ejection. In one example, this activation is due tothe temperature of an ejection bubble of a thermal inkjet. In anotherexample, this activation is due to exposure to the environment duringthe transit from the ejector (770) to the layer of particles (120), forexample, reaction with atmospheric oxygen. In another example, thisactivation is an electrolytic interaction with a thermal inkjet firingresistor.

The heater (790) may be an infrared (IR) heat source. The heater (790)may include a reflective or parabolic dish with a heat generatingelement, for example, a resistor, a series of wires, etc. The heater(790) may be associated with the ejector (770). The heater (790) maymove independently of the ejector (770). The heater (790) may beoperated continuously. The heater (790) operation may be coordinatedwith the activity of the ejector (770). The heater (790) may be locatedin contact with a substrate supporting the layer of particles (120).

The heater (790) may have an associated thermal sensor. In one example,the thermal sensor is a non-contacting thermal sensor. In anotherexample, the thermal sensor is located in and/or on a substratesupporting the layer of particles (120).

The layer of particles (120) may be made of a single type of particles(120) or of multiple types of particles (120). The layer may be uniform.The layer may include areas of variable thickness and/or composition.The layer may be formed on a substrate. The layer may be formed on apreviously formed layer of particles that have been subjected toconsolidation. In some examples the layer is approximately 100micrometers thick. In some examples, the layer is between 20 and 200micrometers thick. In some examples, the layer is between 1 and 1000micrometers thick. In some examples, the particles (120) and metal inthe fluid (780) comprise a common material. In other examples, theparticles (120) are an oxide, a mineral, a polymer, a ceramic, a glass,a metal, a carbide, a nitride, and/or an organic. The ability to use awide range of types and mixtures of particles (120) in the particle(120) layer is a strength of this approach to forming articles.

Within the principles described by this specification, a vast number ofvariations exist. The examples described are examples, and are notintended to limit the scope, applicability, or construction of theclaims.

What is claimed is:
 1. A multi fluid kit for three-dimensional printingcomprising: a first fluid comprising metal nanoparticles; and a secondfluid comprising a destabilizing agent.
 2. The multi fluid kit of claim1, wherein the metal nanoparticles comprise an iron nanoparticle with asilver coating, a nickel nanoparticle with a polyethylene coating,copper nanoparticle with silver coating, copper (II) sulfatepentahydrate, copper (II) nitrate trihydrate, iron (II) sulfateheptahydrate, iron (II) nitrate pentahydrate, copper (II) acetatehydrate, iron (II) acetate, copper oxide, or iron oxide.
 3. The multifluid kit of claim 1, wherein the destabilizing agent comprises sodiumchloride.
 4. The multi fluid kit of claim 1, wherein the first fluidcomprises a first ink vehicle and the second fluid comprises a secondink vehicle.
 5. The multi fluid kit of claim 1, wherein the first inkvehicle and the second ink vehicle are the same or different andcomprise water, co-solvent(s), and/or surfactant(s).
 6. The multi fluidkit of claim 5, wherein: the co-solvent(s) include 2-pyrrolidinone,N-methylpyrrolidone, 1-(2-hydroxyethyl)-2-pyrrolidinone, 1,6-hexanediol,1,5-pentanediol, 2-methyl-1,3-propanediol, triethylene glycol,tetraethylene glycol, tripropylene glycol methyl ether, or combinationsthereof; and the surfactant(s) has a hydrophilic-lipophilic balance(HLB) of less than
 10. 7. A method of forming metal connections in anintermediate three-dimensional part, the method comprising: forming alayer of powder metal particles; selectively applying a fluid comprisingmetal nanoparticles and a destabilizing agent on the layer of the powdermetal particles; and heating or destabilizing the metal nanoparticles inthe fluid to form metal connections.
 8. The method of claim 7, whereinthe destabilizing agent comprises sodium chloride.
 9. The method ofclaim 7, wherein the metal nanoparticles comprise an iron nanoparticlewith a silver coating, a nickel nanoparticle with a polyethylenecoating, copper nanoparticle with silver coating, copper (II) sulfatepentahydrate, copper (II) nitrate trihydrate, iron (II) sulfateheptahydrate, iron (II) nitrate pentahydrate, copper (II) acetatehydrate, iron (II) acetate, copper oxide, or iron oxide.
 10. The methodof claim 7, wherein the powder metal particles comprise stainless steelpowder or copper powder.
 11. A fluid for three-dimensional printingcomprising: metal nanoparticles and a destabilizing agent in ink vehiclecomprising water, co-solvent(s), and/or surfactant(s).
 12. The fluid ofclaim 11, wherein the metal nanoparticles comprise an iron nanoparticlewith a silver coating, a nickel nanoparticle with a polyethylenecoating, copper nanoparticle with silver coating, copper (II) sulfatepentahydrate, copper (II) nitrate trihydrate, iron (II) sulfateheptahydrate, iron (II) nitrate pentahydrate, copper (II) acetatehydrate, iron (II) acetate, copper oxide, or iron oxide.
 13. The fluidof claim 11, wherein the destabilizing agent comprises sodium chloride.14. The fluid of claim 11, wherein: the co-solvent(s) include2-pyrrolidinone, N-methylpyrrolidone,1-(2-hydroxyethyl)-2-pyrrolidinone, 1,6-hexanediol, 1,5-pentanediol,2-methyl-1,3-propanediol, triethylene glycol, tetraethylene glycol,tripropylene glycol methyl ether, or combinations thereof; and thesurfactant(s) has a hydrophilic-lipophilic balance (HLB) of less than10.